Slow decrease of real refractive index of a material in a non-absorbing
spectral region far from regions of strong absorption as the
wavenumber decreases.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 756 (https://doi.org/10.1515/pac-2019-0203)